Split e-invoice exports into a separate server-only subpath

@e-invoice-eu/core pulls in Node-only dependencies (tmp/tmp-promise, for
a LibreOffice conversion path this package never actually takes) that
broke Next.js's client bundle the moment renderInvoiceEInvoice() was
re-exported from the main "." entry — that entry is reachable from a
Client Component (email-templates.ts's Live Preview import chain), so
the bundler tried pulling e-invoice-eu/core in client-side too.

New "@einfach-produktiv/invoicing/einvoice" subpath (package.json
"exports" map + src/einvoice/index.ts) holds only the e-invoice
generation functions — server-only code (API routes, Server Components,
Payload job handlers) imports from there instead. The main "." entry
keeps everything already browser-safe (computeTaxBreakdown, formatters,
InvoiceDocument for Live Preview's <PDFViewer>, the plain PDF renderers).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -6,6 +6,16 @@
   "type": "module",
   "main": "./src/index.ts",
   "types": "./src/index.ts",
+  "exports": {
+    ".": {
+      "types": "./src/index.ts",
+      "default": "./src/index.ts"
+    },
+    "./einvoice": {
+      "types": "./src/einvoice/index.ts",
+      "default": "./src/einvoice/index.ts"
+    }
+  },
   "scripts": {
     "typecheck": "tsc --noEmit",
     "test": "vitest run",